Curbs are values that are set quarterly that affect arbitrageurs and program traders. When curbs go into affect, those groups are not able to sell stocks on a downtick. This curtails their activity and would tend to slow down a market move.

Can you profit from this? Well, you can certainly use this information to your advantage. The profit part is up to you. If the market's selling off hard and the curbs go into effect, you can expect that selloff to slow down and probably stop unless there's some huge driver causing it. I've seen the market hit that point and just chop around. Right now, I believe the curbs are set at 170 points (?) for the Dow and something like 80 points before they are lifted.